Stage IV CMB forecasts for warm inflation

We report forecast constraints on warm inflation in the light of future cosmic microwave background (CMB) surveys, with data expected to be available in the coming decade. These observations could finally give us the missing information necessary to unveil the production of gravitational waves during inflation, reflected by the detection of a non-zero tensor-to-scalar ratio crucial to the B-mode power spectrum of the CMB. We consider the impact of three future surveys, namely the CMB-S4, Simons Observatory, and the space-borne LiteBIRD, in restricting the parameter space of four typical warm inflationary models in the context of a quartic potential, which is well motivated theoretically. We find that all three surveys significantly improve the models' parameter space, compared to recent results obtained with current Planck+BICEP/Keck Array data. Moreover, the combination of ground-based and space-borne (SO+LiteBIRD and CMB-S4+LiteBIRD) surveys tightens the constraints so that we expect to distinguish even better warm inflation scenarios. This result becomes clear when we compare the models' predictions with a ΛCDM+r forecast, compatible with r = 0, in which one of them already becomes excluded by data.